1. Field of the Invention
This invention relates to flexible wiring members such as flexible printed circuits (hereinafter called xe2x80x9cFPCsxe2x80x9d) having excellent flex performance with ease of fabrication.
2. Prior art
Printed wiring boards based on organic substrates are roughly divided into two types; a rigid type having a rigid, copper-clad laminate consisting of glass-epoxy and paper-phenol substrates, and a flexible type having a flexible, copper-clad laminate consisting of polyimide and polyester substrates. Copper foil is mainly employed as a conductive material for the printed wiring boards. Foil products are classified into electrodeposited and rolled foils, depending on the manufacturing processes used.
Of the printed wiring boards, those used for flexible printed circuits (FPCs) are fabricated by laminating a copper foil to a resin substrate and joining the layers with adhesive or with the application of heat and pressure into an integral board. In recent years, multilayer boards known as xe2x80x9cbuilt-upxe2x80x9d boards have come into extensive use as an effective means for high-density packaging or mounting. The copper foil that is used to form components for FPCs is, for the most part, rolled copper foil.
FPCs are largely used in printer heads, hard disk drives, and other components where wiring or conductive connections to movable parts are required. They are subjected to more than a million times of repetitive bending in service. With the recent tendency toward miniaturization and higher performance levels of devices, requirements for flex performance are becoming more severe than heretofore.
The material for copper foil to be used in FPCs is mostly tough-pitch copper (containing 100-500 ppm oxygen). The foil is manufactured by hot rolling an ingot of such material and then repeating cold rolling and annealing alternately until a predetermined thickness is achieved. The rolled copper foil is then plated for surface roughing for enhanced adhesion to a resin substrate. Following the roughing plating, the copper foil is cut into pieces and each piece is laminated to a resin substrate. To join the copper foil and resin together, an adhesive of thermosetting resin, e.g., epoxy, is used. The adhesive is hardened by heating at 130 to 170xc2x0 C. for several hours to several days. Thereafter the copper foil is etched to form various wiring or conductive patterns.
The flex property of a copper foil is markedly improved by recrystallization annealing over that of the foil as rolled. Therefore, the foil is used in the annealed state as an FPC component. The annealing is done either by heat treatment after the roughing plating and cutting into a size or by utilizing the heating at the time of adhering to the resin substrate. The reason for which the annealing is performed during the course of fabrication rather than using an annealed copper foil from the beginning is that, when the copper foil is soft after annealing, it can be deformed or wrinkled upon cutting and laminating to the resin substrate, and a foil hard as rolled is preferred because of the ease of fabrication into an FPC.
For enhanced flex performance of an FPC, improving the flex fatigue property of a rolled copper foil as the starting material is beneficial. The flex fatigue property of an annealed copper foil is improved with the development of its cube texture. In order to help develop the cube texture, it is effective in the copper foil manufacturing process to increase the final rolling reduction ratio and decrease the grain diameter with the annealing immediately before the final rolling (Japanese Patent Application No. 10-101858). Actually, a copper foil manufactured by such a process shows a sharp drop of the softening temperature due to an increase in the plastic strain accumulated by rolling. In extreme cases the foil, even stored at room temperature, can soften after a long period of storage. As noted already, a softened copper foil, if used in the fabrication of an FPC, can cause troubles such as foil deformation and can seriously affect the ease of FPC fabrication. For these reasons it is necessary, when the above manufacturing process is adopted to obtain a copper foil with improved flex property, to heighten the softening temperature of the copper foil to a proper level.
The problem of rolled copper foil softening while being stored at room temperature is pointed out also by Japanese Patent Application Kokai No. 10-230303. The prior application, as a means of controlling the problem, proposes manufacturing a copper foil at a low rolling reduction ratio of 50-90%. In fact, however, a copper foil made at such a low rolling reduction ratio has a seriously deteriorated flex fatigue property, and the proposed means is not acceptable when a copper foil with an excellent flex property is to be obtained. Under the circumstances the present invention aims at raising the softening temperature of a copper foil to a proper level by adding trace amounts of alloying elements to an ordinary tough-pitch copper foil. Various copper alloy foils with additions of many different elements have hitherto been proposed for use in FPCs. However, none of them have proved helpful in solving the problem that the present invention faces; for example, (1) one of the purposes for which an alloying element is added is to inhibit the development of the (100) orientation of a cube texture, and the softening temperature of the resulting alloy foil is unusually high (Japanese Patent No. 2505480), and (2) a large proportion of an alloying element(s) is added to copper to improve its flex resistance (Japanese Patent Application Kokai No. 59-78592).
The object of the present invention is to provide a rolled copper foil for FPCs which combines an excellent flex property with an adequate softening property, by appropriately elevating the softening temperature of a high-flexing rolled copper foil. This eliminates the troubles that can otherwise result from foil softening during storage.
The present invention settles the problems of the prior art and concerns the following:
(1) A rolled copper foil for flexible printed circuits comprising, all by weight, from 0.0100 to 0.0400% of Ag, from 0.0100 to 0.0500% of oxygen, not more than 0.0030% in total of one or more elements selected from the group consisting of S, As, Sb, Bi, Se, Te, Pb, and Sn, and the balance copper, said foil having a thickness in the range of 5 to 50 microns, a half-softening temperature of 120 to 150xc2x0 C., being capable of retaining a tensile strength of at least 300 N/mm2 at 30xc2x0 C., and possessing excellent flex property and adequate softening properties.
(2) A rolled copper foil for flexible printed circuits comprising, all by weight, from 0.0100 to 0.0400% of Ag, from 0.0100 to 0.0500% of oxygen, not more than 0.0030% in total of one or more elements selected from the group consisting of S, As, Sb, Bi, Se, Te, Pb, and Sn, and the balance copper, said foil having a thickness in the range of 5 to 50 microns, the intensity (I) of the (200) plane determined by X-ray diffraction of the rolled surf ace after annealing at 200xc2x0 C. for 30 minutes being I/Io less than 20 with respect to the X-ray diffraction intensity (Io) of the (200) plane of fine copper powder, said foil having a half-softening temperature of 120 to 150xc2x0 C., being capable of retaining a tensile strength of at least 300 N/mm2 at 30xc2x0 C., and possessing excellent flex property and adequate softening properties.
(3) A method of manufacturing the rolled copper foil according to (1) and (2) above by a process which comprises hot rolling an ingot, repeating cold rolling and annealing alternately, and finally cold rolling the work to a foil, the annealing immediately preceding the final cold rolling being performed under conditions that enable the annealed recrystallized grains to have a mean grain diameter of not greater than 20 microns, the reduction ratio of the final cold rolling being beyond 90.0%, whereby an excellent flex property and an adequate softening property are achieved.
When a copper foil is made by a process which involves a high reduction ratio or formation of fine grains to produce a developed cube texture, the foil""s flex fatigue property is improved, but its softening temperature becomes too low. However, judicious control of trace constituents in the material to raise the softening temperature will enable the resulting copper foil to have an adequate softening temperature.
The expression xe2x80x9cadequate softening temperaturexe2x80x9d as used herein may be defined by two conditions:
(1) While the tensile strength of an as-rolled copper foil is in the range of 400-500 N/mm2, the foil should retain a tensile strength of not less than 300 N/mm2 after standing at 30xc2x0 C. for one year.
(2) The copper foil should soften upon heat treatment either after roughing plating and cutting into the desired size, or at the time of adhering to the resin substrate.
The temperature corresponds to the range of 120-150xc2x0 C. in terms of the half-softening temperature obtained by annealing for 30 minutes (the annealing temperature at the point where the tensile strength is intermediate between that before the annealing and that at complete softening).
The present inventors have now found that Ag is the most suitable element for controlling ingot fabrication by melting so that the tough-pitch copper foil manufactured by a process which imparts excellent flex property can have a softening temperature within the target range. The reasons will be explained in more detail below.
(1) Elements less noble than Cu oxidize upon addition to the latter and their concentrations can hardly be controlled in a stable manner. On the other hand, Ag is more noble than Cu and its concentration is easier to control.
(2) Elements added to Cu show much greater softening-inhibiting effects in the state of solid solutions than in the state of precipitates. For example, in the case of an element such as S whose solubility in solid Cu is low and is widely variable with temperature, annealing causes a solid solution/precipitation reaction of the added element, and the reaction materially influences the softening property of the product. In order to control the softening temperature, therefore, it is necessary to adjust not only the amount of the element to be added but also the solid solution/precipitation reaction. The solubility of Ag in Cu is high, on the order of 0.1 wt % at 200xc2x0 C., and, even if it is added in a considerable amount, Ag remains in the form of a solid solution in Cu. This means that adjusting only the amount to be added makes it possible to control the softening temperature.
(3) Generally the electrical conductivity of Cu is decreased by the addition of an element. Ag is exceptional in that its addition does not lower the conductivity of Cu.
(4) Some elements, when added to Cu, hamper the development of a cube texture upon annealing and thereby deteriorate the flex fatigue property of the product. Ag, even when added in an amount of as much as several hundred parts per million, does not inhibit the cube texture development.
As stated above, the present invention is primarily aimed at controlling the softening temperature of the resulting foil within a suitable range through the addition of Ag to ordinary tough-pitch copper. More precise control of the softening temperature is made possible by restricting the contents of impurities other than Ag in the tough-pitch copper below predetermined levels.
The grounds on which various limitations are specified for the rolled copper foil according to the invention will be explained below.
Under the invention the rolled copper foil is intended to retain a tensile strength of not less than 300 N/mm2 continuously at room temperature. More desirably the foil is intended to possess a tensile strength of not less than 300 N/mm2 even after storage for one year at 30xc2x0 C.
Here 30xc2x0 C. corresponds to a temperature above the average annual temperature in Japan. The expression xe2x80x9ccontinuouslyxe2x80x9d used for the storage period before the copper foil is fabricated into FPCs means that the foil is usually stored continuously, for one 2 year at the most. With a tensile strength of 300 N/mm or more, the copper foil has no wrinkling or other trouble during fabrication. There is practically no problem, therefore, when the copper foil is capable of retaining a tensile strength of not less than 300 N/mm2 when allowed to stand at 30xc2x0 C. for one year. Such softening property corresponds, in terms of the half-softening temperature obtained by annealing for 30 minutes, to a temperature of 120xc2x0 C. or upwards.
However, when the half-softening temperature by 30-minute annealing exceeds 150xc2x0 C., the copper foil is sometimes not softened by the heat treatment either after the roughing plating and cutting into a desired size, or at the time of adhering to the resin substrate. That is why the half-softening temperature by 30-minute annealing is specified to be in the range of 120-150xc2x0 C.
If the resulting FPC is to have an enhanced flex fatigue property, the copper foil must itself have an enhanced flex fatigue property. The copper foil is incorporated in a recrystallized state in the FPC, and if the cube texture as a recrystallization texture of pure Cu is allowed to develop, the copper foil attains an improved flex fatigue property. The degree of development of the cube texture that produces satisfactory flex fatigue property is specified to be such that the intensity of the (200) plane determined by X-ray diffraction of the rolled surface is I/I0 greater than 20, preferably I/Io greater than 40.0, with respect to the X-ray diffraction intensity (Io) of the (200) plane of fine copper powder. Here the annealing at 200xc2x0 C. for 30 minutes is conducted to recrystallize the copper foil for the measurement of its X-ray diffraction intensity.
The Ag content is specified to be 0.0100-0.0400 wt % in order to raise the half-softening temperature of the copper foil to an appropriate range, which half-softening temperature has fallen in the course of manufacture for developing its cube texture, to an appropriate range. If the Ag concentration is less than 0.0100 wt %, the half-softening temperature falls below 120xc2x0 C., whereas a concentration beyond 0.0400 wt % raises the temperature above 150xc2x0 C.
S, As, Sb, Bi, Se, Te, Pb, and Sn are impurities usually contained in tough-pitch copper, with relatively large effect upon its half-softening temperature. When the half-softening temperature is controlled by the addition of Ag, the control is made easier by restricting the concentrations of these impurities within low levels. Since these impurities mostly originate from the electrolytic copper used as the material for tough-pitch copper foil, the impurity content of the electrolytic copper should be adjusted so as to control the impurity concentrations in the product.
It is desirable that the amount in total of one or more elements selected from the group consisting of S, As, Sb, Bi, Se, Te, Pb, and Sn should not be more than 0.0030 wt %. If the amount exceeds 0.0030 wt %, the half-softening temperature will vary greatly, even when the Ag concentration remains the same. The half-softening temperature can rise above 150xc2x0 C. depending on the Ag concentration.
Oxygen-free copper of ordinary purity is known to have a remarkably higher softening temperature than tough-pitch copper because of its lower oxygen concentration. Excess oxygen contained in tough-pitch copper forms an inclusion of Cu2O. According to the invention, the oxygen concentration is confined within the range of 0.0100-0.0500 wt %, since the addition of Ag where the oxygen concentration is less than 0.0100 wt % boosts the half-softening temperature beyond 150xc2x0 C., and since the addition of Ag where the oxygen concentration is in excess of 0.0500 wt % increases the Cu2O inclusion, and thereby lowers the flex fatigue property of the product.
As for the thickness of a copper foil, the thinner the foil the better the flex fatigue property because of the lower strain produced around the bend. If the foil is more than 50 microns thick, the desired flex fatigue property will not be attained even when cube texture is developed. Conversely, if the thickness is less than 5 microns, the foil becomes difficult to handle since insufficient strength can cause rupture or other failure. Hence the specified foil thickness range is 5-50 microns.
The copper foil according to the present invention is finished as such by cold rolling to a reduction ratio in excess of 90.0%, following recrystallization annealing under conditions that produce a mean grain diameter of not greater than 20 microns. If the mean diameter upon the annealing that precedes the rolling is more than 20 microns, or if the reduction ratio is less than 90.0%, then I/Io less than 20, and a favorable flex fatigue property will not be achieved. The annealing before the final cold rolling may be combined with hot rolling, in which case it is desirable to adjust the foil grain size as hot rolled to a value not greater than 20 microns.